1. Field of the Invention
The invention relates to aircraft gas turbine engines and, particularly, to turbine vane airfoils of gas turbine engines.
2. Description of Related Art
A typical gas turbine engine of the turbofan type generally includes a forward fan and a booster or low pressure compressor, a middle core engine, and a low pressure turbine which powers the fan and booster or low pressure compressor. The core engine includes a high pressure compressor, a combustor and a high pressure turbine in a serial flow relationship. The high pressure compressor and high pressure turbine of the core engine are connected by a high pressure shaft. High pressure air from the high pressure compressor is mixed with fuel in the combustor and ignited to form a high energy gas stream. The gas stream flows through the high pressure turbine, rotatably driving it and the high pressure shaft which, in turn, rotatably drives the high pressure compressor.
The gas stream leaving the high pressure turbine is expanded through a second or low pressure turbine. The low pressure turbine rotatably drives the fan and booster compressor via a low pressure shaft. The low pressure shaft extends through the high pressure rotor. Most of the thrust produced is generated by the fan. Marine or industrial gas turbine engines have low pressure turbines which power generators, ship propellers, pumps and other devices while turboprops engines use low pressure turbines to power propellers usually through a gearbox.
The high pressure turbine has a turbine nozzle including at least one row of circumferentially spaced apart airfoils or vanes radially extending between radially inner and outer bands. The vanes are usually hollow having an outer wall that is cooled with cooling air from the compressor. Hot gases flowing over the cooled turbine vane outer wall produces flow and thermal boundary layers along outer surfaces of the vane outer wall and end wall surfaces of the inner and outer bands over which the hot gases pass.
There are velocity gradients within the gas flow boundary layer and gas temperature gradients within the thermal boundary layer adjacent to the outer surface of the vane outer wall. The velocity gradient results in shear stresses in the gas flow and forms undesirable aerodynamic drag. The gas temperature gradient results in undesirable heat transfer from the hot gas to the colder outer surface producing unwanted surface heating. It is desirable to reduce the velocity gradients within the gas flow boundary layer to reduce the surface drag and improve the aerodynamic efficiency. It is also desirable to reduce the temperature gradients within the gas thermal boundary layer to reduce the heat transfer for better component life or lower cooling flow requirement for better engine efficiency.